The filoviruses, Ebola and Marburg viruses (EBOV and MARV), are emerging, negative-strand RNA viruses that cause severe viral hemorrhagic fever. The virulence and emerging nature of these zoonotic pathogens makes them a significant threat to human health, potential agents of bioterrorism and NIAID category A priority pathogens. Currently, no approved anti-filovirus therapeutics are available. Filoviral hemorrhagic fever is characterized by uncontrolled, systemic virus replication, excessive inflammation and coagulation, dysregulated dendritic cell (DC) function and lymphocyte apoptosis. We previously identified and structurally characterized filoviral interferon antagonist proteins, including EBOV and MARV VP35, EBOV VP24 and MARV VP40. The VP35s counteract the production of interferon (IFN)-alpha/beta, critical antiviral cytokines. EBOV VP24 and MARV VP40 block the signaling induced by IFN-alpha/beta. We hypothesize that the function of these viral IFN-antagonists will not only counteract the innate antiviral effects of IFN-alpha/beta but, by targeting signaling downstream of pattern recognition, interferon and cytokine receptors, will also be able to disrupt the normal DC maturation process and promote dysregulated DC-T lymphocyte interactions. In this way, the filoviral IFN-antagonists will disrupt both innate and adaptive antiviral immunity. Building on ongoing collaborations between the Basler laboratory at the Icahn School of Medicine at Mt Sinai and the Amarasinghe and Gross laboratories at Washington University, we will characterize the molecular mechanisms of immune disruption by filoviral interferon antagonists. Using in vitro biochemical and structural studies, the Amarasinghe and Gross laboratories at Washington University will define the structural and biochemical basis for how filoviral interferon antagonists interact with host immune system signaling molecules and design loss of function mutants. The Basler laboratory will define the specific signaling pathways affected by each filoviral IFN-antagonist in dendritic cells and macrophages and using the structural and biochemical data provided by Amarasinghe and Gross labs as well as specific knockdown of key signaling molecules will define the impact of filoviral IFN-antagonists on macrophage function and DC cell maturation. The resulting data will generate specific hypotheses regarding the contribution of filoviral IFN-antagonists to EBOV and MARV suppression of DC function (to be tested by Project 2), to promotion of virus replication and induction of IFN and adaptive immune responses in non-human primate models (addressed by Project 3).